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Beneath their still surfaces, the lakes of some Arctic islands may hide the story of the rise and fall of Viking chiefdom.

Historians still aren’t sure exactly what led to the centuries of Viking raiding and expansion, a period politely known as the Scandinavian Diaspora that ran from the late eighth century to the mid-11th. Population pressures and political rivalries probably played a role, but changing climate around the North Atlantic may also have given the Scandinavians a push.

So far, paleoclimate researchers have mostly focused on warmer climates in the Vikings’ destinations, like Iceland, which might have drawn people to settle there. But those who set sail may have been facing trouble with the crops back home thanks to changing temperatures. A team of researchers hope to find some answers in a new series of sediment cores from ancient lakebeds in a remote Norwegian island chain.

“There’s no doubt that there are climate changes that are occurring during that time. The questions are how much of an influence were they on the migrations and settlements at different locations, whether in Norway, Iceland, or Greenland?” said Nicholas Balascio, a paleoclimatologist at the University of William and Mary.

He and his colleagues spent the summer in the Lofoten Islands, a low-lying archipelago off the coast of Norway, well above the Arctic Circle. It’s the perfect place to study how Iron Age people responded to climate change. Lofoten is so far north that it’s on the edge of the farmable world, so small shifts in summer temperature make a notable difference in the growing season here.

The islands were once the center of a major Viking chiefdom (a term for both a style of government and an individual government), based in the village at Borg, and a jumping-off point for Viking settlement of Iceland. Finding the links between the Lofoten islands and migration westward is easy. The main island still holds the remains of the chieftain’s longhouse at Borg and a cluster of Viking Age boathouses. Documents from the 11th century show people from Lofoten—including the last chieftain himself—were among the first few hundred Viking settlers in Iceland. From there, the lake cores will only add more to the story, helping scientists better understand shifts in climate, population, and land use over a period of 3,000 years.

Rise and fall of a Viking Chiefdom

In its Viking Age heyday, Borg was a center of wealth and power. Its chieftain lived in the largest Viking longhouse that’s ever been found in northern Norway, unearthed by archaeologists in the late 1980s. People caught cod by the boatload here, then dried the fish to create a highly portable, mostly non-perishable food product called stockfish. It doesn’t sound very appetizing today, but it was a major export to the coastal cities of mainland Europe—and a staple for Viking raiders and settlers.

“When the Vikings went on their various raids and travels out into the North Atlantic, they usually took with them stockfish as provisions,” said Stephen Wickler, an archaeologist at the University of Tromsø. Archaeological evidence reveals a booming trade in stockfish during the Viking age. Lofoten’s cold climate was perfect for air-drying cod without using expensive salt, and the growing demand for cheap protein-rich food would have given the chieftain of a prime fishery like Borg a lot of political and economic clout.

Yet Borg attained that wealth and power despite being almost too far north to make a living at all. The warm current of the Gulf Stream kept the islands warm enough for farming, but only barely. The people who lived in Lofoten would have grown grains—mostly barley—and raised livestock, but a slight shift in summer temperatures could spell failure for that year’s crops, forcing people to find another way to survive, move elsewhere, or starve. Because the local economy was so sensitive to small climate fluctuations, geoscientist William D’Andrea and his colleagues at Columbia University's Lamont-Doherty Earth Observatory think this might be the place to look for links between climate change and a shift in the islands’ Viking Age economy.

“The people who were living on Lofoten were living in the most marginal environment of all of the Viking Kingdoms throughout Scandinavia. They were living the farthest north, and they were spending a lot of time on boats because they were engaged in cod fisheries,” said D’Andrea. “I always think about the experience up there as maybe being representative of their exploration spirit. They were ready to go looking for new things, and they were ready to make a living in a marginal environment.”

But the prosperous chiefdom had basically collapsed by the early 12th century. The boathouses, which had seen use off-and-on since the fifth or sixth century, (the Iron Age Norwegians built things to last) were abandoned by this time. Archaeological work on the chieftain’s huge longhouse shows that late in the Viking Age, it too was abandoned, or at least occupied by fewer people.

“The suggestion has been made through evidence from other places that the chieftain may have emigrated to Iceland,” said Wickler. “And whether he was forced to do so by competition with other chiefdoms to the south or whether that was a decision he made of his own free will is difficult to get at.”

The power of the Viking chiefdoms was beginning to consolidate in the hands of a few of the largest, which would eventually coalesce into the kingdoms of the medieval period. Those changes have long been blamed for the decline of chiefdoms like Borg—powerhouses in their day, but not powerful enough to keep up with the emerging kings.

The chieftain didn’t emigrate alone. Several Lofoten islanders’ names appear in the Landnámabók, a list of the original families who migrated to Iceland from Norway in the ninth century. That population shift undoubtedly accelerated the collapse of the once-prosperous and powerful chiefdom at Borg.

But teasing out cause and effect is challenging—did the migration trigger the collapse, or did people migrate because the chiefdom was collapsing? That’s part of what the team hopes to puzzle out of the mud at the bottom of lakes and harbors on Lofoten.

“In the region that we’re studying, where there’s some evidence that some of the people who migrated from Lofoten did settle in Iceland, we want to better understand if there was climate or environmental changes that played a part in that history as well,” said Balascio.

Enlarge/ This mud is from Glastonbury Festival 2016, so much different cultural studies happen there.

Ian Gavan/Getty Images

Answers in the mud

When you step into a lake and feel mud oozing between your toes, you’re actually stepping in soil that washed in from the surrounding landscape, carrying with it traces of what that landscape is like. Chemicals called sterols, from human and animal feces, record how densely populated the area is. Plant pollen records whether the surrounding area is forest, farmland, or meadow. Traces of charcoal and ash record cooking fires or slash-and-burn land clearing.

Over time, that soil stacks up in layers to form a long, detailed record. To read that record, scientists drill down several meters and bring up a long cylinder of layered earth encased in a Plexiglass tube: 3,000 years of history, in this case. Back in the lab, they’ll analyze this research season’s 14 core samples for all sorts of data about what the lakes and their surroundings were like in the past and how they changed.

“All of our mud is still in tubes, but the main objective is to ask how population size has changed throughout the Viking Age and how has local climate changed throughout the Viking Age,” said D’Andrea. He and his colleagues will compare that information with evidence from archaeological sites around Lofoten—like the boathouses and the longhouse, along with hundreds of other sites—to see if changes in climate or sea level seem linked to changes in the local economy or population.

Part of those answers may be waiting in sediment cores from five lakes in a wide valley of farmland running the length of the main island. That’s where D’Andrea and his colleagues hope to learn more about when the Lofoten islanders first took up agriculture and how their farming habits changed over the years. In an earlier study, human and livestock populations seemed to fluctuate along with summer temperatures, and the team hopes the new cores will hold further clues about how climate impacted the economy of the islands.

In cores taken from two lakes closer to the coast, the team will look for indications of ancient sea-level changes that probably explain the abandonment of the boathouses.

At the beginning of the Iron Age, the sea level in the Lofoten Islands was higher than it is today. That’s because geological forces have lifted the islands upward for at least the last 6,000 years. Sea level had already begun to fall by the end of the Viking Age, and receding shorelines would eventually have cut off the boathouses’ access to the harbor, making launching fishing boats from Borg exceedingly difficult. The change in elevation could also have let more freshwater from the islands drain into the harbor to create a brackish estuary that would actually have started to freeze over in the winter, effectively stranding the ships.

D’Andrea and his team hope the sediment cores will tell them exactly how much the Borg shoreline here has moved since the days of the Vikings. Remains of aquatic life, long buried in the mud at the bottom the estuary that once formed one of Borg’s harbors, will help researchers understand when the estuary shifted from a saltwater harbor to a freshwater lake.

Together, Lofoten’s lakes could preserve indications of how the people of the Lofoten Islands responded to shifting climate and changing sea levels during the course of the Iron Age. They may tell the researchers, for instance, if shifts in sea level led people to shift to greater dependence on agriculture or if colder summers led people to shift their reliance from farming to fishing.